MXPA97004027A

MXPA97004027A - Method for operating a cycle-combination energy plant

Info

Publication number: MXPA97004027A
Application number: MXPA/A/1997/004027A
Authority: MX
Inventors: Mack Shelor F
Original assignee: Wartsila Diesel Inc
Priority date: 1994-12-01
Filing date: 1997-05-30
Publication date: 1998-02-01

Abstract

A method is described for using the exhaust of an internal combustion engine (1) in a combined cycle power plant, where the quality and distribution of the exhaust to the space of the boiler (3) of a power plant that it generates steam, they are controlled to achieve greater system efficiencies. External air is mixed only with the exhaust portion passing through the burner gates (8) as secondary or higher level combustion gas. The rest of the exhaust is provided to the boiler space by a different route than through the burner (20). Higher overall system efficiencies are achieved where the amount of external air mixed with the exhaust portion passing through the burner is such that the mixture contains approximately the minimum amount of oxygen required for a complete and stable combustion of the selected fuel, where a substantial percentage of the total exhaust is directed towards the space of the boiler by a different route than through the burner, and where the amount of fuel is sufficient to reach a desired inlet temperature to the boiler on its combustion.

Description

METHOD FOR OPERATING A COMBINED CYCLE ENERGY PLANT I. BACKGROUND OF THE INVENTION The present invention relates to the use of an engine exhaust and internal combustion in combined cycle power plants. More particularly, the present invention relates to achieving greater efficiencies of the system by controlling the quality and distribution of the exhaust into the boiler space of a typical steam-generated power plant. With regard to the design of the power plant, efficiency provides a useful measure of system operation. As the power plant converts energy from one form to another, losses are inevitable. Where the designer reduces those losses, or even transforms byproducts or the disposal of certain processes into available energy sources, the overall efficiency of the system will naturally increase. It is known in the art that efficiencies in power generation can be achieved by recycling the exhaust of the internal combustion engine as a secondary combustion gas and as an ignition or sub-ignition air in a power plant generated by steam ignited by typical coal. . In my United States of America Patent Number 4,928,635, I disclose said system. One of the objects of this invention was to make the exhaust energy available to generate steam. Therefore, efficiencies were achieved simply by converting what would otherwise be waste into productive energy. At this time, I realized that it was necessary to raise the temperature of the exhaust in order to produce a high quality steam. I suggested that re-ignition of an exhaust mixture containing approximately 13 percent oxygen and air previously heated as a secondary combustion gas would be an appropriate method to achieve that result. I also suggested that the total flow of the exhaust to the boiler should preferably be approximately 40 to 70 percent of the total gas flow to the boiler. Upon further investigation, I discovered that greater overall system efficiencies could be achieved by controlling the amount of oxygen in key locations inside the burner, or directing substantially higher proportions of the exhaust into the boiler space directly, as opposed to directing it as a secondary or higher level combustion gas, thus lowering the amount of supplementary ignition required in the boiler. The total flow of the exhaust to the boiler should constitute a higher percentage of the total gas flow to the boiler than I had previously suggested, in order to take full advantage of the thermal energy of the exhaust, and to eliminate the introduction towards the boiler, at least as far as possible, gases at a lower temperature. The method of the present invention reflects that discovery.

II. COMPENDIUM OF THE INVENTION Where internal combustion engine exhaust is used to generate steam for process requirements or for the production of electricity, it may be necessary to increase the exhaust gas temperature from the internal combustion engine to levels that are appropriate for the production of high quality steam. The re-ignition of the exhaust - burning additional fuel in its presence - makes this result a reality. The combustion of the fuel raises its temperature and that of the surrounding and downstream exhaust as well as that of any other gases present. The amount of fuel that must be burned to raise the exhaust temperature depends, of course, on the type of fuel that is being used. It also depends on the total amount to which the temperature must be raised, and the initial temperature of the gas. Greater system overall efficiencies will be realized where the heat added to the system is minimized to meet the steam conditions, in another way that is not provided by the exhaust, since this heat represents the fuel that must be burned. The amount of heat that must be added to the system usually increases as the amount of gas in the system increases. The fuel must be burned in the presence of oxygen. In general, it is necessary to provide external air containing a percentage of oxygen to the burner, such as a secondary combustion gas, thus ensuring that there is sufficient oxygen available to achieve a complete and stable combustion of the fuel. However, since the external air must necessarily enter the system, its temperature must also rise to satisfy the steam conditions. The more external air is used, the more heat must be added to the system in the form of burned fuel. Where the exhaust is used as a secondary combustion gas, its higher temperature relative to external air results in a reduction in the amount of heat that must be added to satisfy the steam conditions. Although the exhaust usually contains some oxygen, it may be insufficient to achieve a complete and stable combustion of the fuel. In accordance with the foregoing, some external air must be mixed with the exhaust to bring the level of oxygen within the mixture to an amount sufficient to achieve a complete and stable combustion of the fuel provided through the burner.

The amount of oxygen needed to achieve complete and stable combustion, of course, will also depend on the volatility of the fuel selected or already available. The elevation of the oxygen level for the entire cross section of the exhaust would require the addition of a substantial amount of external air. To reduce the amount of external air entering the boiler, external air is mixed only with the portion of the exhaust that passes through the burner gates as a secondary or higher combustion gas. The rest of the exhaust is provided to the boiler space by a different route than through the burner. The highest overall system efficiencies are achieved where the amount of external air mixed with the portion of the exhaust that passes through the burner is such that the mixture contains approximately the minimum amount of oxygen required for a complete and stable combustion of the selected fuel , where a substantial percentage of the total exhaust is directed towards the space of the boiler by a different route than through the burner, and where the amount of fuel is sufficient to achieve a temperature of entry to the desired boiler on its combustion. Greater overall system efficiencies can be achieved by practicing the invention independently of the initial oxygen content of the exhaust. In the same way, higher efficiencies can be achieved regardless of the specific fuel selected. The invention provides a method of operation that by its nature is flexible, adapting to any potential sources of energy that may be available. The existing combined cycle generation plants can be modified at a reasonable cost to allow the operation of the method. In the same way, whenever an electric power plant with existing steam generation can be adapted for combined cycle work, the method can be practiced. These and other advantages of the present invention, as well as a preferred method for practicing the invention, will be better understood in view of the accompanying Figure and the following discussion.

III. BRIEF DESCRIPTION OF THE DRAWING The Figure is a diagram that shows the most basic elements common to the typical combined cycle generation plants.

IV. DETAILED DESCRIPTION OF THE PREFERRED WAY TO PRACTICE THE INVENTION To demonstrate the preferred method of the invention, we now turn to Figure. The Figure shows the most basic elements common to typical combined cycle generation plants. The plant employs at least one internal combustion engine 1. The engine can be any internal combustion engine, but preferably it is a diesel engine. This engine can be adapted to burn natural gas, light fuel oil, or heavy fuel oil, among other fuels. The branches 2 and 2 'direct the exhaust from the engines to an electrical power plant with typical steam generation, all the elements of which are not shown in the Figure for clarity. In the Figure a boiler space 3 is shown, around whose periphery steam tubes 4 are arranged. Inside the steam tubes 4 water or steam circulates around the periphery of the space of the boiler 3. It is in this interface that exchanges the heat between the space of the boiler 3 and the steam of the steam pipes 4. The exposure to the hot gases inside the space of the boiler 3 causes the temperature of the steam to rise inside the steam pipes 4. Then the superheated steam is circulated to a steam turbine generator (not shown), where most of the thermal energy of the steam is transformed into electricity. Only a portion of the exhaust enters the space of the boiler through one or more burner exits. As shown in the Figure, branches 5 and 6 direct each, a portion of the exhaust to the burner 20. The branch 7 directs the rest of the exhaust to the space of the boiler 3 directly, deviating from the burner 20. This portion of the exhaust enters the space of the boiler 3 through the hatches or nozzles 8. The burner 20 includes a primary outlet or nozzle 21. The primary outlet 21 is adapted to deliver fuel to a combustion zone 30. The fuel may be coal, either micronized or pulverized, liquefied bituminous fuel, heavy fuel oil, Residual oil, ormulsion, or any other suitable fuel. The selection of an appropriate burner depends on the choice of fuel, the nature of the electric power plant that generates steam, and the given steam conditions. Commercially available burners, such as those manufactured by Babcock &; ilcox, are suitable where the burner provides the mixture of fuel and oxygen, maintaining appropriate oxygen levels for the combustion of the selected fuel at the tip of the burner, and delivering secondary or higher combustion gases. The Babcock & Wilcox XCL, as well as adaptations and subsequent generations of such burners, are most preferred. Where the fuel is coal, the average oxygen level at the tip of the burner is preferably about 14.5 percent. Where heavy fuel oil or natural gas is used, the level is preferably about 14.1 percent and 13 percent, respectively. Preferably, the fuel is mixed with a sufficient amount of air to carry or transport the fuel. The benefits can be improved by dedicating to maintaining a reducing atmosphere in a portion of the combustion zone 30, and allowing the combustion to proceed in stages where secondary, tertiary, or higher-level combustion gas streams provide the necessary oxygen to complete the successive stages of combustion. The exhaust directed by the branches 5 and 6 eventually enters the space of the boiler through the burner outputs 22 and 23. Preferably, the exhaust flow of the burner is at most 40 percent of the total exhaust flow that eventually it will be delivered to the boiler space 3. More preferably, the burner exhaust flow is approximately 20 percent of the total exhaust flow that will eventually be delivered to the boiler space 3. The burner exhaust flow works as the exhaust gas. secondary and tertiary combustion, which is delivered in circumferential rings around the outlet of the primary burner 21, and provides shape, stability, and oxygen to the flame. The oxygen content of the exhaust directed by the branches 5 and 6 is usually insufficient to achieve a complete and stable combustion of the fuel. Additional oxygen must be supplied to the exhaust flow. This oxygen is supplied by mixing the external air with the exhaust directed by the branches 5 and 6. Preferably, the external air is preheated by passing it through a steam coil air heater 40 before directing it through the branch 41 to the burner 20. Pre-heating reduces the amount of heat that must be added subsequently to raise the air temperature, and consequently, reduces the amount of fuel that must be burned. An optimum efficiency will be achieved where the amount of external air that is mixed with the exhaust flow is such that it provides the minimum complement of oxygen necessary to achieve a complete and stable combustion of the fuel, which generally translates into the minimum amount of external air necessary to achieve the same purpose. The exhaust directed by the branch 7 deviates from the burner 20. The diverted exhaust flow enters the space of the boiler 3 downstream of the combustion zone 30, and is preferably delivered to the space of the boiler 3 through the outlets or nozzles 8 in a wall or walls of the boiler space 3. After the diverted exhaust flow enters the space of the boiler 3, it is mixed with the combustion products and the exhaust flow of the burner (now at a temperature elevated). On the mixture, the gases tend towards a uniform average boiler inlet temperature. Preferably, the diverted exhaust flow is at least about 60 percent of the total exhaust flow to be delivered to the boiler space 3. More preferably, the diverted exhaust flow is at less than 80 percent of the flow of exhaust. total exhaust that will be delivered to the boiler space 3. Optimal efficiencies will be achieved where the average boiler inlet temperature is the minimum necessary to achieve the given steam conditions. The method of the invention can be further demonstrated with reference to a simple system comprising the following components and limitations or operating characteristics: (1) a diesel engine generator VASA 18V46 in full load with fuel oil No. 6; (2) a boiler fired with fuel oil No. 6. Fresh combustion air is added to the fuel to maintain 14.6 percent oxygen (base in wet weight) in the windbox of the burner. The burner ignites to maintain the minimum excess of 10 percent oxygen at the burner outlet, resulting in an ignition temperature of approximately 1537 ° C leaving the burner. The temperature of the wind box is maintained at approximately 295 ° C; (3) The generation of steam is based on an economizing outlet temperature of 149 ° C, without being lowered. The steam is generated under feed water conditions at 91 kg / cm2 / 510 ° C; (4) the fuel inlet is based on fuel oil No. 6, base LHV, 17,233 BTU / 0.4536 kg; (5) environmental conditions: 30 ° C, relative humidity of 60 percent, sea level. Typical operating parameters for this system are provided in the following Table: Typical Operational Parameters of Diesel Combined Cycle Ignited with Oil No. 6 Therefore, for given steam conditions, optimum efficiencies are achieved where the addition of fuel and air is minimized, or, conversely, where a substantial portion of the exhaust from the internal combustion engine enters the boiler space by a route different than through the burner. The proposed system can be more clearly understood if the boiler is treated as a separate component of the internal combustion engine. The exhaust contributes to a fixed amount of heat for the boiler, and fuel is added to this fixed level, to enable the boiler to produce steam of a given quality. Based on the amount of fuel required, whose quantity is necessarily a function of the quality and nature of the fuel, an amount of oxygen must be made available in and around the combustion zone to achieve a complete and stable combustion of the fuel, such as shown in the Table, the point of greatest apparent efficiency of the boiler is from the point where the minimum amount of fuel is added to satisfy the steam conditions. In this example, the minimum boiler inlet temperature (maximum deviation) is approximately 665 ° C, providing an apparent boiler efficiency of 150 percent. With the goal of providing an efficient combined system for large power generation using diesel engines as the base, and retaining the flexible fuel characteristics of diesel combined cycle systems, a preferred configuration with which the method can be practiced employs six dieseis VASA 18V46 in combination with a heat recovery steam generator of three overheating pressures. Regardless, recognizing that the diesel exhaust provides a fixed amount of recoverable heat, and that fuel can be added to the exhaust in order to overcome the tightening points of the boiler for each steam cycle, it is clear that an entire set of potential sizes of the power plant using steam turbines with reheating, or without overheating. Heavy fuel oil at dieseis is provided at 885.8 MBTU / H / 17233.0 BTU / 0.4536 kg. The total output of the diesel generator is 90.7 MW. The burner exhaust flow is 123,061.68 kg / H at 348 ° C. The diverted exhaust flow is 492,337.44 kg / H, or approximately 80 percent of the total exhaust flow that will enter the boiler space, at 348 ° C. The external air at 31 ° C and with a relative humidity of 80 percent, is previously heated to 148 ° C, and is delivered to, and mixed with, the exhaust flow of the burner at 21,886.2 kg / H. Heavy fuel oil No. 6 is supplied to the burner at 231.1 MBTU / H / 17233.0 / 4536 kg. Alternative fuels include natural gas or light fuel oil. The use of orimulsion or coal, of course, would require some change in the portion of the plant's steam system. In general, where more difficult fuels are involved, the boiler can not be used at three pressures, and a two-pressure system can be used. Particularly dirty fuels may require that specific environmental control measures be employed after the portion of the plant's steam system. Under these conditions, a boiler inlet temperature of 665 ° C is reached, and a gross heat index of 7016.6 BTU / KWH (lowest heating value, gross output of the plant). The gross output of the plant and the net output are 130.6 MW and 126.7 MW, respectively, operating the steam turbine at 102.55 kg / 537 ° C / 537 ° C to produce 39.9 MW. Where the diverted flow to 60 percent decreases, a higher gross heat index of 7172.51 BTU / KWH (lower heating value, gross output of the plant) is reached. The gross output of the plant and the net output are 160.0 MW and 155.2 MW, respectively, operating the steam turbine at 102.55 kg / 537 ° C / 537 ° C to produce 69.3 MW. The higher fuel consumption and external air account for the difference in efficiency. In relation to the previous configuration, the burner exhaust flow has increased to 247,380 kg / h at 348 ° C. The diverted exhaust flow has decreased to 371.184 kg / H at 348 ° C. The external air at 31 ° C and with a relative humidity of 80 percent is preheated to 148 ° C, and is entered into, and mixed with, the exhaust flow of the burner at an increased rate of 43.772.4 kg / H. Heavy fuel oil No. 6 is provided to the burner at the increased rate of 482.2 MBTU / H / 17233.0 BTU / 0.4536 Kg. It should be appreciated that the method of the present invention can be performed in a variety of ways, only some of which have completely described in the above. Without departing from the spirit or essential character, the invention can be realized in other ways. The foregoing should be considered in all respects only as illustrative and not restrictive, and accordingly, the scope of the invention is described by the appended claims rather than by the foregoing description. All changes that fall within the meaning and range of equivalence of the claims are covered within their scope.

Claims

1. A method for operating a combined cycle power plant comprising an internal combustion engine, a burner, and a boiler space, the method comprising: directing a first portion of the exhaust from the internal combustion engine to the space of the boiler by a different route than through the burner; directing fuel through a primary outlet of the burner in an amount sufficient to reach an entry temperature to the desired average boiler on its combustion; providing a second exhaust portion from the internal combustion engine to eventually be directed through at least one burner outlet different from the primary burner outlet, - mixing an amount of air with the second exhaust portion, such that the Air / exhaust mixture contains approximately the minimum oxygen level appropriate for a complete and stable combustion of the fuel; directing the air and exhaust mixture through at least one burner outlet; and burn the fuel.

2. The method of claim 1, wherein the first portion of the exhaust is at least about 60 percent of the entire exhaust directed into the space of the boiler. The method of claim 2, wherein the first portion of the exhaust is up to about 80 percent of the entire exhaust directed into the space of the boiler. 4. The method of claim 1, wherein the first portion of the exhaust is at least approximately 54 percent of the total mass of all the gas that entthe space of the boiler. The method of claim 4, wherein the first portion of the exhaust is up to about 76 percent of the total mass of all gas entering the boiler space. The method of claim 5, wherein the internal combustion engine is a diesel engine. The method of claim 6, wherein the air is heated before being mixed with the second portion of the exhaust. The method of claim 7, wherein the fuel is mixed with a quantity of carrier air before combustion. The method of claim 8, wherein a reducing atmosphere is maintained in a portion of a combustion zone. The method of claim 8, wherein the combustion of the fuel proceeds in stages. The method of claim 8, wherein no escape is directed from the internal combustion engine through the primary burner outlet. The method of claim 5, wherein the first portion of the exhaust entthe space of the boiler downstream of a combustion zone. 1

3. A method for operating a combined cycle power plant comprising an internal combustion engine, a burner, and a boiler space, the method comprising: directing a first portion of the exhaust from the internal combustion engine to the space of the boiler by a different route than through the burner; direct the fuel through a primary outlet of the burner; providing a second portion of the exhaust from the internal combustion engine to eventually be directed through when there is a burner output different from the primary burner outlet, - mixing an amount of air with the second exhaust portion, such that the Air / exhaust mixture contains approximately the minimum oxygen level appropriate for a complete and stable combustion of the fuel; direct the mixture of air and exhaust through at least one burner outlet, - and burn the fuel, where the first portion of the exhaust is at least approximately 54 percent of the total mass of all gas entering the space of the boiler. The method of claim 13, wherein the first portion of the exhaust is up to about 76 percent of the total mass of all the gas entering the space of the boiler. 15. The method of claim 14, wherein the first portion of the exhaust is at least approximately 60 percent of the entire exhaust directed into the space of the boiler. The method of claim 15, wherein the first portion of the exhaust is up to about 80 percent of the entire exhaust directed into the space of the boiler. 17. The method of claim 13, wherein the fuel is approximately the minimum amount of fuel needed to reach an entry temperature to the desired average boiler upon its combustion. 18. A method for operating a combined cycle power plant comprising an internal combustion engine, a burner, and a boiler space, the method comprising: directing a first portion of the exhaust from the internal combustion engine into the space of the boiler by a different route than through the burner; directing the fuel through a primary outlet of the burner in an amount sufficient to reach an entry temperature to the desired average boiler on its combustion; provide a second portion of the exhaust from the internal combustion engine to eventually be used as a secondary or higher level combustion gas, - mix an amount of air with the second portion of the exhaust, such that the mixture of air and exhaust contains approximately the minimum oxygen level appropriate for a complete and stable combustion of the fuel; provide the air and exhaust mixture as secondary or higher level combustion gas; and burn the combuetible. The method of claim 18, wherein the first portion of the exhaust is at least about 60 percent of the entire exhaust directed into the space of the boiler. The method of claim 19, wherein the first portion of the exhaust is up to about 80 percent of the entire exhaust directed into the space of the boiler. The method of claim 18, wherein the first portion of the exhaust is at least about 54 percent of the total mass of all gas entering the boiler space. The method of claim 21, wherein the first portion of the exhaust is up to about 76 percent of the total mass of the gas entering the space of the boiler. The method of claim 22, wherein the mixture of air and exhaust is provided as a secondary and tertiary combustion gas. The method of claim 23, wherein the amounts of oxygen in the secondary and tertiary combustion gases are not the same.